libxl: events: Permit timeouts to signal ao abort

libxl: events: Permit timeouts to signal ao abort

The callback functions provided by users must take an rc value.  This
rc value can be ERROR_TIMEDOUT or ERROR_ABORTED.

Users of xswait are now expected to deal correctly with
ERROR_ABORTED.  If they experience this, it hasn't been logged.
And the caller won't log it either since it's not TIMEDOUT.
Luckily this is correct, so we can just change the doc comment.

Currently nothing generates ERROR_ABORTED; in particular the timeouts
cannot in fact signal abort requests.

There should be no publicly visible change except that some error
returns from libxl will change from ERROR_FAIL to ERROR_TIMEDOUT, and
some changes to debugging messages.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v4: ABORTED not CANCELLED.